ASTRONOMERS DISCOVER FLUORIDE MOLECULES IN INTERSTELLAR SPACE

A team of astronomers from the United States and Germany
has discovered trace amounts of hydrogen fluoride gas in the
near vacuum of interstellar space, using the European Space
Agency's Infrared Space Observatory satellite, ISO, which was
launched in November 1995.

Although approximately one hundred different kinds of
molecule have been detected in interstellar space over the past
30 years, the discovery of hydrogen fluoride marks the first
time that a molecule containing fluorine has been detected in an
interstellar gas cloud. The discovery was reported in an article
that is to appear in the Astrophysical Journal Letters.

The astronomers searched for hydrogen fluoride molecules
within a giant cloud of interstellar gas located near the centre
of the Milky Way galaxy. The new observations were carried
out in March with the Long Wavelength Spectrometer, one of
four instruments on board ISO. Looking in the far-infrared
region of the electromagnetic spectrum, the astronomers
observed the telltale signature of absorption by trace amounts
of hydrogen fluoride gas.

"Because the Earth's atmosphere is completely opaque to far
possible only from space," said Professor David Neufeld of the
Department of Physics and Astronomy, Johns Hopkins
University, Baltimore, leader of the team that reported the
finding. "The ISO satellite has opened up an exciting new
window on the universe by allowing us to observe at far-infrared
hydrogen fluoride molecules absorb radiation is approximately
one eighth of a millimetre, much larger than the wavelength of
visible light but much smaller than the wavelengths typically
used for radio and television communications.

In concentrated liquid form, hydrogen fluoride - or hydrofluoric
acid as it is known when dissolved in water - is familiar to
laboratory chemists as an extremely dangerous and corrosive
acid that dissolves glass and severely burns human tissues. The
gas cloud in which hydrogen fluoride molecules were
discovered lies approximately 20,000 light years from Earth, in
the Southern constellation Sagittarius. Known to astronomers
as Sagittarius B2, the gas cloud is composed primarily of
hydrogen molecules. As in other clouds of interstellar gas, the
environment in Sagittarius B2 is very extreme by terrestrial
standards, with temperatures less than minus 220 Celsius, and
pressures more than one hundred million million times smaller
than the atmospheric pressure on Earth. And although the
hydrogen fluoride is less than one thousand millionth as
abundant as the hydrogen, the sensitivity of the ISO
spectrometers made its detection possible.

"This discovery gives us the opportunity to study the chemistry
of fluoride molecules in the frigid conditions that characterise
the near vacuum of interstellar space," says Neufeld. "One of
the key questions is how these molecules were formed. Our
analysis suggests that the hydrogen fluoride we detected was
produced by direct chemical reactions between fluorine atoms
and hydrogen molecules. Unlike most atoms, fluorine atoms
are extremely reactive and attack the relatively inert hydrogen
molecules that are the principal constituent of the interstellar
gas. The result is hydrogen fluoride."

The members of the team that made the discovery of hydrogen
fluoride are Neufeld, Professors Jonas Zmuidzinas and Thomas
Phillips of the California Institute of Technology, and Dr Peter
Schilke of the Max-Planck Institute for Radio Astronomy in
Bonn, Germany. The participation of Neufeld, Zmuidzinas and
Phillips as guest observers on ISO was supported by the
National Aeronautics and Space Administration, NASA.

The Infrared Space Observatory is a satellite built and launched
by ESA, with instruments supplied by Principal Investigators in
France, Germany, the Netherlands and the United Kingdom.
ISO is operated by ESA with the support of NASA.

The Long-Wavelength spectrometer was built by a consortium
of scientists and engineers from Canada, France, Italy, the UK
and the USA, led by Professor Peter Clegg of Queen Mary and
Westfield College, University of London, England.